The present invention relates to a zoom lens system that has small constraint on back focus and that is suitable for use with compact cameras. More particularly, the present invention relates to a zoom lens system that has an exceptionally wide view angle (approximately 37.degree. in terms of half view angle) in consideration of its use with compact cameras, that is small in size and that yet features a high zoom ratio of 2.5 and more.
Various types of zoom lens systems have heretofore been known for use with compact cameras. Zoom lenses consisting of three lens units or more with a zoom ratio exceeding 2 are categorized as follows:
(i) Four-unit zoom lens system comprising, in order from the object side, a positive lens unit I, a negative lens unit II, a stop diaphragm, a positive lens unit III, and a negative lens unit IV, all the lens units being moved independently of each other towards the object (this type includes a zoom lens where some of the four lens units are moved in unison). Examples of such lens systems are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-43115, No. SHO 63-159818, and No. SHO 63-157120.
(ii) Three-unit lens system comprising a positive lens unit I, a negative lens unit II (which has a negative lens unit IIa disposed towards the object, a positive lens unit IIb disposed towards the image, and a stop diaphragm therebetween and which is equivalent to the lens unit II and the lens unit III of the above-described four-lens unit zoom system) and a negative lens unit II (which is equivalent to the lens unit IV of the four-lens unit zoom system), all the lens units being moved independently of each other towards the object. An example of this zoom lens system is disclosed in Japanese Patent Laid-Open Publication No. SHO 63-153511.
Examples of zoom lens systems that consist of three or more lens units and that have zoom ratios of less than 2 (approximately 1.5-1.6) are described in Japanese Patent Laid-Open Publications No. SHO 60-57814 (a four-unit type) and No. SHO 762-78522 (a three-unit type). The arrangement of the lens units and stop diaphragm in these systems are the same as in the systems (i) and (ii) described above.
(iii) Three-unit zoom lens systems comprising, in order from the object side, a positive lens unit I, a positive lens unit II (as in the system (ii), this positive lens unit II comprises, in order from the object side, a negative sub-unit IIa and a positive sub-unit IIb but no stop diaphragm is included), a stop diaphragm and a negative lens unit III, with all the lens units being moved toward the object. An example of this type of lens system is disclosed in commonly assigned Japanese Patent Application No. SHO 63-225294).
(iv) Effectively a four-unit zoom lens system that is the same as the system (iii) described above except that the sub-units IIa and IIb are moved independently of each other, e.g., as in Example 3 in the specification of commonly assigned Japanese Patent Application No. SHO 63-225294.
The above-described conventional zoom lens systems have their own problems. In the system (i), all of the four lens units have to be moved independently of each other, so many cams must be used in a restricted small space, causing difficulty in designing an effective mechanical composition for the lens barrel.
In the systems (i) and (ii), the stop diaphragm is disposed between the sub-units IIa and IIb (in the case of the four-unit type, the stop diaphragm is disposed between the lens units II and III); however, a shutter block must be disposed between the two lens units which are subject to substantial deterioration in performance due to any fabrication errors, so the overall lens arrangement will become complex, making it difficult to insure high mechanical precision. In addition, the high optical sensitivity of either system has made it difficult to achieve stabilization in optical performance. Furthermore, the compactness of those lens systems is still insufficient for use with compact cameras.
With a view to solving the aforementioned problems, the present inventors previously proposed the systems (iii) and (iv) described in Japanese Patent Application No. SHO 63-225294. However, those systems as well as systems (i) and (ii) feature a half view angle of only about 30.degree. at the wide-angle end, and their main object is to provide an improved telephoto effect by increasing the focal length at the narrow-angle end. However, many users require a wide enough angle to permit taking scenic pictures. To those users, the half view angle of about 30.degree. at the wide-angle end is insufficient, but conventional zoom lens systems have not fully met the requirement for wide angles and high zoom ratios.
In short, conventional zoom lens systems for use with compact cameras are basically of a telephoto type over the entire zooming range, and it has generally been held that the requirement for achieving a wider angle and higher zoom ratio at the same time is very difficult to meet in such zoom lens systems for use with compact cameras that are basically of a telephoto type.
A further problem with a high zoom ratio zoom lens system for use with compact cameras is that the more compact they are, the more abrupt the increase in the amount of defocusing (or focus movement) that occurs in response to the change in the distance between lens units. The focusing position is sensitive to temperature, so that at elevated temperatures even a slight increase in the distance between lens units will cause a substantial change in the focusing position in the negative direction (towards the lens). This problem has been particularly noticeable in the case where one of the lenses used is a plastic lens which is highly susceptible to temperature changes.